The present invention relates to gas treating methods, in particular a method for cooling a gas below the onset of solidification of a liquid in the gas without ending up with solidified liquid plugging the system.
U.S. Pat. No. 3,904,494 entitled "Effluent Gas Recycling and Recovery in Electrolytic Cells for Production of Aluminum from Aluminum Chloride", issued Sept. 9, 1975 in the names of Stanley C. Jacobs and Ronald C. Schoener discloses a system for treating the gas emitted from a cell in which aluminum is being produced by the electrolysis of aluminum chloride. The system includes a demister. The gas coming off the cell is cooled until certain constituents in the gas condense in the form of a mist, i.e. liquid droplets suspended in the gas. The function of the demister is to remove this mist from the gas. The gas then proceeds from the demister to, for example, a shell-and-tube heat exchanger where the gas is cooled further to begin collection of solid aluminum chloride. The nature of aluminum chloride is such that it can move from the gaseous state directly to the solid state under appropriate pressure conditions, for example, atmospheric pressure.
A problem that existed for a long time in experiments on this system was that, no matter how seemingly perfect the demister was made, there always seemed to be some liquid that did not get trapped in the demister. This liquid would settle out in the conduit joining the demister with the shell-and-tube heat exchanger, and it would then drain from the conduit down into the top of the shell-and-tube heat exchanger, thence to begin plugging up the heat exchanger tubes as the liquid became solid under the temperature conditions existing in the shell-and-tube heat exchanger. Considerable time and money were spent in trying to solve this problem by perfecting the demister.